Process of producing protected metal articles



Patented Aug. 7, 1945 PROCESS OF PRODUCING PROTECTED METAL ARTICLES James M. Schoonmaker, Jr., Sewickley, and Franklin Stockton, Ooraopolis, Pa., assignors to Standard Steel Spring Company, Coraopolis, Pa, a corporation of Pennsylvania No Drawing. Application December 13, 1940,

Serial No. 370,004

BClaims. (01. 204-37) This invention relates generally to the protection of metals from attack by corrosive and similar agents, and has particular reference to the formation of protective layers of metal applied for the purpose indicated.

One object of the invention is to provide a way of producing a protective coating which has the external appearance and chemical characteristics of tin, but which will be much more eiiective from a protective standpoint than the present commercial coatings of corresponding thickness composed only of tin. As an important result, less tin is needed, and the new'coating is nevertheless better without sacrificing anything of valve arising from the inherent characteristics of tin. Another object of the invention is to provide a protectively coated metal article which can be used wherever the present tin plates can be used and with much better results. In addition, the new coatings may be appreciably thinner and cost less than tin coatings aflording the same amount of protection, while the new coatings of comparable cost and thicknesses give much greater protection than the corresponding tin coatings.

In commercial operations as at present conducted, tin coatings are mostly applied in two ways. The one in most general use is what is known as thehot dip process. In some instances, when hot dipping is employed and heavy tin coats are desired, the material or fabricated articles are simply hand dipped in molten tin, but tin plates and strips from which articles are to be fabricated are usually tinned in a machine comprising a pot of molten tin through which the material is fed. The mechanism for feeding the material usually includes rollers at the exit side of the pot between which the material passes,

these rollers acting to smooth out and reduce the tin coating to the desired thickness. Commercial tin plate for cans and the like usually has a tin coating about 0.000090" thick, although the thickness may be varied within a somewhat narrow range according to customer requirements, by suitable adjustments of the mechanism.

The other process in use commercially is an electroplating one, the tin bein deposited electrolytically from a bath. The amount of tin deposited may vary from a mere flash to any desired practical thickness.

The protective value of tin coatings depends largely upon their continuity. The prior tin coatings are not continuous, as pin holes and other defects exposing the metal base are sure to be present. Even very minute openings through the tin give a chance for corrosive reagents to enterand the base metal will begin at once to corrode or rust when exposed or used in environments where moisture is present, this efiect being particularly rapid when the moisture contains or is accompanied by corrosive reagents.

Applicants process yields a coating which is in itself novel, and which is not affected by contact with such reagents. In addition, the new coating is continuous and therefore gives substantially perfect protection to the metal to which the coating is applied.

This application is a continuation in part of United States application Serial No. 237,924, filed October 31, 1938, by the present applicants.

In making a protective coating according to the invention,'the surface of the article is first cleaned and prepared in any of the usual ways and a primary metallic coating is then applied to that surface. The preferred metal for this primary coating is nickel. Nickel is easily applied and, like tin, has a high protective value if it is continuous, i. e., free from pin holes. It is preferred to apply the nickel by electro-deposition. After the nickel has been applied, the article is rinsed and a tin coating is then applied electrolytically. The thicknesses of the coatings andtheir relative thicknesses may be varied within reasonable limits, but, because of the cost factor, coatings which are as thin as the proposed use permits, are the most desirable. For the best results the nickel coating should be thick enough to leave in contact with the face of the article a layer of nickel which is unaffected by the alloying treatment, explained later, forming a part of the process.

Tin coatings so thin that the tin was as low as V 1 percent by volume, of the nickel gave a noticeable improvementin resisting corrosion, but in commercial practice the amount of tin should be at least 10 percent by volume of the nickel. The tin seldom needs to exceed a volume equal to the volume of the nickel, or. to state the proportions in other terms, the tin should in commercial practice, run from a thickness of about one tenth of the thickness of the nickel up to a thickness about equal to the thickness of the nickel. The relative proportions or thicknesses stated are, however, not to be considered as limitations upon the method or the resulting article.

One embodiment using coatings of 0.000030" of nickel and 0.000030" of tin has proven very satisfactory and the first cost, including subsequent treatments hereinafter described, is about the same as the cost of 0.000090" hot dipped tin.

The treated nickel-tin coat, however, gives an average of more than five times the protection afforded by the tin coat. I

Another embodiment which has been proven to be just as eillcient as the 0.000090" tin from a rotective standpoint is 0.000020" nickel and 0.000010" tin. The cost of'this particular combination of coatings, including the cost of subsequent treatments, is about one-half of the cost of hot dipped tin, but it gives at least as much protection in all cases as the tin coat, with an average eii'ective life which is much higher than that of the tin. V

In another application of the invention, nickel 0.000050" thick and tin 0.000040" thick has been employed, this giving the same total thickness (0.000090") as the usual hot dipped tin. The

cost of this coating. including the cost'of subsequent treatments, will be a trifle more than that of the hot dipped coating of tin only. but when the coating has been completed as hereinafter described, it will give many times the protection afforded by the tin alone.

In all embodiments of the invention the double coated article is subjected to a heat treatment. This varies somewhat according to the thicknesses of the coatings and according to the mass of the article to which the coating has been applied. It is impossible to state here the exact temperature and time for heat treatment for all kinds of articles where the masses of the base metal vary, but the examples given above may be considered as applied to such sheet gauges as are usually employed for tin plated articles. In the example first mentioned, that is, 0.000030" nickel and 0.000030" tin, the coated article should be subjected to a temperature'oi' about 400 F. for from ten to fifteen minutes, the shorter period usually being sufficient. The time will vary somewhat according to conditions, but the fifteen minute application of'heat need not very often be exceeded. In the other example given, that is, 0.000020" nickel and 0.000010" tin, a temperature of 400 F. should be applied for fifteen seconds.

In the other illustrative example, that is, 0.000050" nickel and 0.000040" tin, experience indicates that a temperature of 400 F. for fifteen minutes gives the best results, although about the same results have been obtained by heating for two hours at a temperature of 300 F.

These coatings, heat treated as above indicated, have all stood up against corrosion for many times the length of time which coatings of tin alone of the same thicknesses were able to stand when subjected to the same corrosive conditions.

The heat treatment results in the formation of a nickel-tin alloy layer between and joining together the nickel and the tin and also causes the tin to infuse into or penetrate any pores or crevices in the nickel even though they may be microscopic size. Enough tin will, however, be left unchanged on the surface of thearticle to give a bright tin appearance and the completed article can, if desired, be burnished or polished. The larger part, preferably, and always some of the nickel is left unaffected by either the heat or the tin, thereby giving an eflicient coating of nickel only attached to and covering the base metal.

Any suitable apparatus for applying the heat may be employed. It is preferred, however, to

use a furnace or heater in which the temperature and atmosphere may both be controlled.

Preferably all oxygen should be excluded from the heating chamber.

The coating so far discussed has been selected because of the commercial situation with respect to present commercial tin plates. However, the invention can be used to apply coatings heavy enough to compete with block or hand dipped tin where the service requirements are unusually severe. For example, a coating of 0.000500" nickel and 0.000500" tin heated fort'wo hours at a temperature of 300 F. showed no signs of even initial failure after being subjected to accelerated corrosion tests for about 1500 hours and. other coatings of 0.000200" nickel and 0.000200" tin were still in good condition from a protective standpoint at 2800 hours accelerated tests, although some only of them showed insignificant indications that the corrosion factor had begun to reach the base metal at isolated points; but even so, they were in substantially perfect condition so far as protecting the base metal is concerned. Other tests showed that the heat treated 0.000050" nickel and 0.000040" tin coating had a protective life of the coating which was many times the life of the 0.000090" hot dipped tin.

Specimens coated only with tin, 0.000090" thick, when subjected to the same tests, show pronounced evidences of failure in from three to twenty-four hours.

The examples specifically mentioned illustrate the invention satisfactorily but they are, as before indicated generally, not intended to be more than representative of the practice. In making such nickel tin coatings it is desirable to use enough tin to serve as a diffusing agent or form a nickeltin alloy in the surface portion only of the nickel and leave enough unaffected tin on the exposed surface to give a bright tin appearance. This appearance is, however; not important for some uses and it is not the intention to say that the completed article must always look light bright tin. As will be apparent from all of the foregoing, the inventioii is capable of various modifications and changes, all coming within the scope of the claims which follow.

What is claimed is:

l. The method of protecting a ferrous article against corrosion which consists in depositing a continuous coating of nickel of substantially uniform thickness on the article, then applying to the nickel a coating of tin, and then heating at a temperature above 350 F. and below the. melting point of tin for a period of time sufficient to diffuse at least a part of the tin into the outer portion only of the nickel.

2. A method of protecting ferrous articles against corrosion which consists in applying to the article an adherent coating of nickel of substantially uniform thickness, covering the nickel with an adherent coating of tin which is substantially uniform in thickness and having such a total volume as will be diffused into the nickel suiliciently to leave only a part of tin on the surface of the nickel when the coatings are heated for about two hours at a temperature between about 300 F. and about 350 F., and then heating the coatings for the time and at the temperature stated.

3. The method of protecting a metal article against corrosion which consists in depositing a coating of nickel on the article, then depositing on the nickel a coating of tin which is less in total volume than the volume of the nickel, and then heating the article at a temperature appreaching but not exceeding the melting point of tin and for a period of time aumoient only to diffuse the larger part of the tin into the outer portion only oi the nickel coating.

4. The method of protecting a ferrous article against corrosion which consists in depositing a coating of nickel on the article, then depositing on the nickel a coating oi, tin which is under about 90 percent by volume of the volume of the nickel, and then heating the article 'at a temperature below the melting point of tin for a period of time 'suflicient to dii'luse some of the tin into the nickel and leave the rest of the tin in a. continuous coating on the outer face 01- the nickel.

5. The method oi protecting a metal article against corrosion which consists in depositing a continuous coating of nickel or substantially uniform thickness on the article, and sealing the outer face of the nickel by ilrst applying a coating of tin and then heating at a temperature under the melting point oi. tin and for a period of time suilicient to diilfuse part of the tin into the outer portion of the nickel while leaving a continuous film of tin on the face of the nickel,

the total volume or the tin being under about 50 percent oi the volume of the nickel.

6. The method of protecting the surface of a ferrous article against corrosion which consists in electro-depositing on said surface a layer of nickel, electro-depositing on said nickel a layer of tin and then heat treating the coated article at a temperature above 350 F. and below the melting point of tin and holding the article at said temperature long enough to produce a protective layer oi nickel-tin alloy sealing pores in the nickel while leaving unalloyed tin on the exterior oi the article.

JAMES M. SCHOONMAKER, JR. FRANKLIN STOCKTON. 

